An ultraviolet sensor has been widely used as a flame sensor for a fire alarm device or a combustion monitoring device of a burner, and as an ultraviolet detection device for measuring the amount of ultraviolet radiation in an outdoor environment. In recent years, an ultraviolet sensor has also been expected to be applied to an optical communication device.
As this type of ultraviolet sensor, hitherto, a sensor using a diamond semiconductor or a SiC semiconductor as a sensing material has been known. However, these diamond semiconductors and SiC semiconductors have defects that the ability of materials to be processed is inferior and the materials are expensive.
Hence, in recent years, the oxide semiconductor which is easy in material processing and relatively inexpensive receives attention, and research on and development of an ultraviolet sensor formed by joining a p-type semiconductor layer to an n-type semiconductor layer in the form of a hetero junction by using these oxide semiconductors.
For example, in Patent Document 1 is disclosed an ultraviolet sensor 106 including a ZnO layer 101 of an n-type oxide semiconductor, a (Ni, Zn) O layer 102 of a p-type oxide semiconductor disposed in contact with the ZnO layer 101, a first terminal electrode 103 electrically connected to the ZnO layer 101, and a second terminal electrode 105 electrically connected to the (Ni, Zn) O layer 102 through a conductive layer 104 made of a transitional metal oxide or the like.
In this Patent Document 1, when the sensor is irradiated with ultraviolet light in a direction of an arrow a, and a depletion layer formed at a junction between an n-type ZnO layer 101 and a p-type (Ni, Zn) O layer 102 is irradiated with ultraviolet light, carriers are excited, and a photocurrent is generated, and thereby; ultraviolet light can be detected.
However, since the carrier concentration of the (Ni, Zn) O layer 102 is extremely lower than the carrier concentration of the ZnO layer 101 and the specific resistance of the (Ni, Zn) O layer 102 cannot be adequately lowered, only a slight photocurrent is generated even though the ZnO layer 101 is irradiated with ultraviolet light. Furthermore, since the slight photocurrent is almost consumed by internal resistance of the (Ni, Zn) O layer 102, it is a fact that the photocurrent cannot be actually detected as a current value.
Therefore, in Patent Document 1, as shown in FIG. 7, a first terminal electrode 103 and a second terminal electrode 105 are formed at the surface side of the ZnO layer 101 and the back side of a conductive layer 104, respectively, and a power source circuit 107 is externally provided, and the intensity of ultraviolet light is detected as changes in a resistance value. That is, when ultraviolet light is irradiated, a voltage is applied to the first terminal electrode 103 and the second terminal electrode 105, and at a resistance 109 disposed in parallel with a power source 108, changes in resistance are measured, and the intensity of ultraviolet light is detected by changes in a resistance value.
Patent Document 1: Japanese Patent Publication No. 3952076